Switching DC-to-DC converters having a multiphase coupled inductor topology are described in U.S. Pat. No. 6,362,986 to Schultz et al., the disclosure of which is incorporated herein by reference. These converters have advantages, including reduced ripple current in the inductors and the switches, which enables reduced per-phase inductance or reduced switching frequency over converters having conventional multi-phase DC-to-DC converter topologies. As a result, DC-to-DC converters with magnetically coupled output inductors achieve a superior transient response without an efficiency penalty compared with conventional multiphase topologies. This allows a significant reduction in output capacitance resulting in smaller, lower cost solutions.
DC-to-DC converters are often used in applications where the load may vary considerably as a system operates. For example, the processor of a modern notebook computer may demand tens to more than one hundred amps of current when performing processor-intensive computation at maximum clock rate, while the processor needs much less current, possibly only a few milliamps, when the system is idle. When a DC-to-DC converter is designed to power such a processor, the inductors, capacitors, and switching transistors of the converter are typically designed to handle the maximum sustained current required by the processor without overheating. There are many other applications for power converters where converter load current levels may vary over time. Variation between maximum and minimum load current of factors of hundreds to thousands are not unusual.
DC-to-DC converters typically operate in Continuous Conduction Mode (CCM), where switching and AC current related losses do not scale down with decreasing load current. Therefore, switching and AC current related losses may become a significant part of the total power absorbed by the converter when the load current is small. Since many systems spend considerable portions of their operating lifetime operating at low power levels, they may waste considerable energy over their lifetimes. It is especially important in battery powered systems that DC-to-DC converters operate at high efficiency over the entire range of possible output power demand to optimize battery life.
Accordingly, it may be desirable to operate DC-to-DC converters in Discontinuous Conduction Mode (DCM) under light load conditions to reduce switching and AC current related losses. Additionally, one or more phases of a multiphase DC-to-DC converter may be shut down during light load conditions to reduce switching and AC current related losses. Examples of DC-to-DC converters including coupled inductors that may operate in DCM and/or shut down phases during light load conditions may be found in U.S. Pat. Nos. 7,317,305 and 7,548,046 to Stratakos et al., each of which is incorporated herein by reference.